Late Cretaceous Tectonic History of the Sierra-Salinia-Mojave Arc As

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B02204, doi:10.1029/2003JB002845, 2004 Late Cretaceous tectonic history of the Sierra-Salinia-Mojave arc as recorded in conglomerates of the Upper Cretaceous and Paleocene Gualala Formation, northern California Ronald C. Schott1 and Clark M. Johnson Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin, USA James R. O’Neil Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA Received 14 October 2003; revised 5 November 2003; accepted 17 November 2003; published 11 February 2004. [1] Upper Cretaceous and Paleogene conglomerates in the Gualala basin of northern California likely preserve the westernmost record of sedimentation in California during the interval between cessation of Sierran batholith magmatism and inception of Neogene transform activity on the San Andreas fault system. Detailed chemical and H, O, Sr, Nd, and Pb isotope analyses of conglomerate clasts, as well as U/Pb zircon ages, identify two source types: (1) evolved granite, like those found in the Salinian block or eastern Sierra Nevada batholith, and (2) primitive oceanic arcs that were restricted to the western portions of the Mesozoic arc system. The provenance of the Gualala clasts can be entirely explained through derivation from the Sierra-Salinian-Mojave portion of the Mesozoic Cordilleran arc, and do not support models for extensive large-scale translation of the basin from low latitudes along a pre-Neogene transform fault system. Contemporaneous deposition of the contrasting lithologies in the Gualala basin is interpreted to reflect tectonic juxtaposition of the source terranes approximately 80 m.y. ago. Tectonic processes responsible for juxtaposition of the two source regions in the Late Cretaceous most probably involved zones of lithospheric weakness that later localized the San Andreas fault system in the Neogene. Chemical, isotopic, and age data tie the Gualala basin to North American continental basement in the Mojave-Salinian segment of the Mesozoic Cordilleran arc and imply minimal pre-Neogene translation of the basin contrary to the low latitude origin for the basin inferred from previous paleomagnetic and paleontologic studies. INDEX TERMS: 1035 Geochemistry: Geochronology; 1040 Geochemistry: Isotopic composition/chemistry; 1065 Geochemistry: Trace elements (3670); 8105 Tectonophysics: Continental margins and sedimentary basins (1212); KEYWORDS: isotope, provenance, cordillera Citation: Schott, R. C., C. M. Johnson, and J. R. O’Neil (2004), Late Cretaceous tectonic history of the Sierra-Salinia-Mojave arc as recorded in conglomerates of the Upper Cretaceous and Paleocene Gualala Formation, northern California, J. Geophys. Res., 109, B02204, doi:10.1029/2003JB002845. 1. Introduction North America. More controversial are interpretations of thousands of kilometers of displacement suggested by some [2] Sedimentary basins in orogenic belts record tectonic paleomagnetic and paleontologic studies [e.g., Kanter and events that may manifest themselves on the surface, which Debiche, 1985; Elder et al., 1998]. A significant unresolved may be distinct from those that are preserved in deep crustal aspect of the latter hypotheses is the documentation of environments. In some cases, sedimentary basins may faults along which such large displacements could be record the effects of large-scale translation of exotic terranes accommodated. [e.g., Cowan, 1994; Dickinson and Butler, 1998]. Many [3] The Gualala basin offers a unique opportunity to exotic terranes of western North America have demonstra- address the tectonic evolution of the Cordillera of California ble and well-accepted post-Cretaceous displacements of and of the role of the faults that offset it. Currently situated tens to hundreds of kilometers with respect to cratonic adjacent to the San Andreas fault in northern California (Figure 1), the basin is clearly offset from its location of origin by motion along the San Andreas fault system in the 1Now at Department of Geology and Physics, Lake Superior State University, Sault Sainte Marie, Michigan, USA. Neogene [e.g., Ross, 1984; Graham et al., 1989; Powell, 1993]. Pre-Neogene translation may have occurred as well. Copyright 2004 by the American Geophysical Union. Some workers have suggested that the basin was originally 0148-0227/04/2003JB002845$09.00 located hundreds of kilometers south of its current position B02204 1of22 B02204 SCHOTT ET AL.: CRETACEOUS HISTORY OF THE GUALALA BASIN B02204 Figure 1. Current location in California of the displaced Salinian block (stippled pattern) and Gualala basin (GB), as well as the Eagle Rest Peak (ERP) mafic basement terrane and its offset counterparts at Gold Hill (GH) and Logan Quarry (LQ). Faults: SAF, San Andreas fault; GF, Garlock Fault; SGHF, San Gregorio-Hosgri fault; RRF, Reliz-Rinconada fault; SNF, Sur-Nacimiento fault. Cities: SF, San Francisco; LA, Los Angeles; FR, Fresno; BK, Bakersfield. and adjacent to the northern Salinian block and southern San Peak (ERP) and its outliers at Gold Hill (GH) and Logan Joaquin Valley [e.g., Wentworth, 1966; Ross et al., 1973; Quarry (LQ), rocks that have been debated as possible Kistler et al., 1973; James et al., 1993; Schott and Johnson, source materials for mafic conglomerate clasts at Gualala. 1998a, 1998b, 2001], while others place it thousands of Identifying these sources would independently constrain kilometers south of its current position [e.g., Kanter and offset on the San Andreas fault in central California [Ross, Debiche, 1985; Elder et al., 1998]. Detailed discussions of 1970; Ross et al., 1973; Kistler et al., 1973; James et al., the contrasting hypotheses for the origin of the basin are 1993]. presented by Burnham [1998] and Schott [2000]. [4] Because the sedimentary record of the Gualala basin spans almost 30 m.y. during the Late Cretaceous and early 2. Depositional Setting and Age Paleogene, sedimentary debris within it record pre-Neo- [5] Upper Cretaceous and early Tertiary sedimentary gene tectonic events. In this paper we present new geo- rocks in the Gualala basin include conglomerates, sand- chemical and isotopic data that bear on the provenance of stones and mudstones that were deposited as a turbidite fan- conglomerate clasts from the Gualala Formation. These channel complex at bathyal depths in a forearc tectonic clasts are of two types, gabbroic and rhyolitic/granitic, in setting [Wentworth, 1966; Loomis and Ingle, 1994]. places mixed together in the same beds. The present study Wentworth [1966] assigned Upper Cretaceous and lowest encompasses both the gabbroic clasts that have been the Paleocene rocks to the Gualala Formation and subdivided it focus of study by previous workers [Ross, 1970; Ross et into two members on the basis of conglomerate clast al., 1973; Kistler et al., 1973; James et al., 1993], as well and sandstone composition (Figure 2). The Stewarts as rhyolitic and granitic clasts [Schott and Johnson, 1998a], Point Member is characterized by felsic volcanic and plu- which have received less attention. The focus here is on the tonic clasts in a matrix of K-feldspar arkose, whereas the Late Cretaceous to Paleocene section and is complemented Anchor Bay Member contains a mafic clast assemblage in a by previous work on the overlying Eocene German Rancho plagioclase arkose (<5% K-feldspar) [Wentworth, 1966]; the Formation [Schott and Johnson, 2001]. New data are also two members are interfingered and hybrid felsic-mafic presented for the mafic basement terrane at Eagle Rest conglomerate clast suites occur near their mapped contacts. 2of22 B02204 SCHOTT ET AL.: CRETACEOUS HISTORY OF THE GUALALA BASIN B02204 Figure 2. Generalized geologic map and stratigraphic section of the Gualala Formation (modified from Wentworth et al. [1998]). Conglomerate sample locality key: A, Black Point (KCGR clasts only); B, Smuggler’s Cove (KCGR clasts only); C, Sea Ranch Stable/Pebble Beach (interfingered and hybrid conglomerate beds, KCGR and JOGD clasts); D, Anchor Bay (JOGD clasts only); E, Havens Neck (JOGD clasts only). Localities and lithologies for clasts are given in Table 1. The Upper Cretaceous section sits structurally above spilitic chemical analyses (Table 3), and isotopic compositions basalt of uncertain tectonic affinity [Phillips et al., 1998], (Table 4). and Wentworth et al. [1998] conclude that the contact between these units is a low angle or ‘‘detachment’’ fault. 3.1. Upper Cretaceous Conglomerate Clasts Stratigraphically above the Gualala Formation lies the [8] Upper Cretaceous conglomerate clasts can be broadly Paleocene to lower Eocene German Rancho Formation subdivided into two suites that are judged to be continental (Figure 2), another section of turbidites that are dominated or oceanic on the basis of their age and isotopic composi- by K-feldspar arkose and contain conglomerates of marked- tions. This source distinction corresponds to the composi- ly different composition [Schott and Johnson, 2001]. tional and mapped field designation of the Stewarts Point [6] Microfossils and sparse macrofossils constrain a and Anchor Bay Members of the Gualala Formation as Campanian age for the lowest Stewarts Point strata and a originally delineated by Wentworth [1966]. Clasts of conti- Maastrichtian age for the majority of the section [Loomis nental origin (designated KCGR) are most commonly and Ingle, 1994; Elder et al., 1998; Wentworth et al., 1998]. rhyolitic or granitic (often porphyritic) and have a range Conglomerates

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